1. Field of the Invention
The invention relates to a rotary-anode X-ray tube, comprising a sleeve bearing which serves to journal the rotary anode and comprises a stationary bearing portion and a bearing portion which is rotatable about an axis of rotation, the two bearing portions cooperating with one another via a lubricant and comprising bearing surfaces which extend perpendicularly to the axis of rotation in order to take up axial bearing forces, said bearing surfaces changing over into external surfaces which form a lubricant gap between the bearing portions and one of which is provided with a groove pattern.
2. Description of the Related Art
A rotary-anode X-ray tube of this kind is known from EP-OS 378 273 (=U.S. Pat. No. 5,068,885). The groove pattern is composed of converging grooves. These grooves do not extend linearly, but as lengths of arc of two oppositely directed (logarithmic) spirals; therefore, such a sleeve bearing is also referred to as a spiral-groove bearing. When the bearing rotates in the prescribed direction of rotation, a lubricant pressure is built up at the area in which the pairs of grooves meet, said lubricant pressure determining the bearing capacity of the bearing. The dimension of the cylindrical lubricant gap between the two axial bearings in the known bearing (approximately 0.5 mm) is large in comparison with the distance between the bearing surfaces at the area of the groove pattern (typically 0.02 mm), so that the lubricant gap serves as a lubricant reservoir wherefrom lubricant can be transported to the area of the groove pattern.
It has been found that the bearing capacity of the bearing in respect of axially acting bearing forces may deteriorate after operation for several hours in unfavorable operating conditions.